Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 170446
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1146 PM EST Sat Feb 16 2019


.AVIATION...

Lake Strato cu continues to be advected into Se Mi within northeast
flow off Lake Huron. During the course of the early morning, the low
level flow will veer more easterly. Upstream satellite show plenty
of strato cu across Lake Erie and northern Ohio. There is also a push
of drier air across Ontario, supporting another area of clearing. In
short, there are likely to be intervals of MVFR based strato cu with
periodic clearing during the course of the morning. Some better
probabilities for more prolonged clearing of the low clouds look to
take hold late Sunday morning into early Sun afternoon as
strengthening easterly flow temporarily advects some drier air into
Se Mi. Light snow will then overspread the area late Sun afternoon
(after 20Z), resulting in lowering cigs and vsby. The peak snow will
occur across the area between 20Z Sun afternoon and 03Z Sun evening,
with an inch or two of snow expected.

For DTW...The period of peak snowfall will be a little earlier than
locals to the north, likely in the 20Z to 01Z time frame. An inch or
two of snow will be possible through this time period.

.DTW THRESHOLD PROBABILITIES...

* Moderate in ceilings below 5000 ft overnight and Sunday morning.
  High late Sun afternoon and Sun night.

* High that precip will fall as all snow Sunday afternoon and
  evening.

&&

.PREV DISCUSSION...
Issued at 722 PM EST Sat Feb 16 2019

UPDATE...

Although the late afternoon clearing trend across the Saginaw Valley
has persisted and expanded into Flint and the eastern thumb, there is
evidence that the Lake Huron moisture plume will continue to
influence a good portion of the area during the late evening into the
overnight as the low level flow continues to veer more northeasterly.
The low level flow will actually veer easterly toward Sun morning.
The extent of low clouds across Lake Erie and northern Ohio suggests
that the more pessimistic cloud forecast across metro Detroit/Ann
Arbor and points south looks reasonable. Farther north, while there
may be some additional periods of lake clouds, observational evidence
suggests there will at least be significant periods of clear skies.
The forecast will be updated to have a more graduation in sky cover,
from most clear across Midland/Bay counties to most cloudy along the
M-59 corridor and points south. The nighttime temperature forecast
will also be adjusted accordingly; a little warmer in south and
colder in the north based on nighttime sky coverage.

PREV DISCUSSION...
Issued at 332 PM EST Sat Feb 16 2019

DISCUSSION...

Widespread snow showers are winding down this afternoon as
increasing subsidence and diurnal heating erode moisture depth and
inhibit dendrite production within the extensive plume of lake
stratocu. Lowered highs a second time for this afternoon given
observational evidence that boundary layer flow is already beginning
to veer to a more favorable overlake fetch that will ensure some
persistence of clouds for most or all of the aftn and likely filling
in again tonight. Went ahead and raised lows 3 to 5 degrees tonight
as well due to clouds.

Focus for Sunday will be on potential for widespread accumulating
snow as a number of the NWP suite which are struggling to play catch-
up to the next surge of a very energized subtropical jet are
catching up to the NAM family which remains qualitatively unchanged
for its last several runs. To lesser consequence, the NWP have also
universally overestimated confluent flow over NW Canada which has
been modeled to impinge on the Great Lakes. The combined result of
these factors is the general trend in the guidance toward a faster
and more poleward solution for Sunday.

Strong isentropic ascent will overspread the CWA 12-18z Sunday as
the LLJ evolves northward almost completely orthogonal to the upper
jet. Meaningful upper jet dynamics, however, will be long gone early
in the day as the early push of support will mainly serve to
invigorate the LLJ and help saturate the column while producing just
a few isolated snow showers or virga during the morning. In general,
expect increasing probability of virga with northward extent during
the afternoon as well. The earliest onset is forecast to occur at
the nose of the incoming mid-level theta-e ridge for Detroit and
points south no earlier than 1 PM local time.

Cross-sections indicate there really is not much to this
event from a forcing perspective. It will simply be strong WAA
serving to completely erode static stability between about 800mb and
500mb over the CWA. There will exist a tendency for organization
along the aforementioned convergent theta-e ridge which is currently
modeled to push up to about the M59 corridor in time for the evening
commute before the pivot out of the area begins. H85 to H7 layer
average specific humidity ranging from 2.5 g/kg in the M59 counties
to well over 3 g/kg near the Ohio border will certainly be capable
of supporting locally higher amounts in should a greater degree of
organization materialize. For now, a general 1 to 2 accumulation
potential M-59 south looks good with less to the north given delayed
onset and dry air. Further, once low-level saturation is complete
over the northern CWA, the strong jet will only be a detriment as it
forces the aggressive dry slot over area after 00z both diminishing
ascent and deep layer moisture quality. The potential for any higher
end accums would exist within a narrow band perhaps as wide as a
county or less with an upper end potential of a localized 4" given
limited forcing and duration. The 12z NAM12 gives an overzealous nod
to this potential as the solution implodes due to convective
feedback within this favored corridor, while the 12z NAMNest offers
a more subdued response and provides the general framework for the
afternoon forecast package. In fact the 18z HRRR appears to have
latched onto this potential as well. The existence of neutral
stability with pockets of shallow conditional instability and
ongoing trend of the NWP struggling to capture just how much energy
will be zipping by Lower Michigan on Sunday suggest a bit of caution
when it comes to expressing confidence in both location and
magnitude of snow, hence the extended paragraph dedicated to
outlining what currently presents as a lower probability outcome of
something in excess of 2 inches. Worth a note that the shallow 0-
0.5km unstable layer over Lake Huron will likely further slightly
enhance the moisture field near the ground. Progged thermodynamic
profiles will support event-average snow ratios near climo, around
13:1 - less to the north. Lingering boundary layer moisture and
descending dry air aloft will support low ratio snow/flurries
lingering into Monday morning.

Active jet stream will remain directed across the CONUS through the
majority of next week and SE Michigan`s placement on the poleward
side of the jet max among plenty of confluence aloft will result in
mostly quiet conditions. The exception will be during the midweek
period when potent Pacific shortwave energy results in amplification
of the longwave locally and brings a good chance for precip via
isentropic ascent. Current trends suggest light snow starting
Wednesday morning and lasting into the evening, with a chance for
some rain to mix in on the tail end as a surge of warmer temps
reaches the area. Temperatures will take on a rising trend through
the week, with highs increasing from the 20s on Tuesday to the upper
30s/lower 40s by Saturday. Lows in the teens and 20s will be likely.
The next chance for rain comes Saturday with a deep trough digging
through the central CONUS.

MARINE...

Light winds are expected into tonight as high pressure builds across
the area. Moderate easterly winds will then develop on Sunday as low
pressure tracks just south of the Ohio Valley. Winds will back to
the northeast Sunday night and north on Monday as this low pressure
continues to the east. Wind gusts will peak late Sunday into Sunday
night at 25 to 30 knots and then decrease from Monday into Tuesday
as another high pressure system builds into the region as winds back
further to the northwest. Easterly winds will then return by midweek
as the next low pressure system encroaches on the region. No
headlines are anticipated at this time as wind gusts remain below
gale criteria and nearshore areas remain largely ice covered with
limited wave action.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....SC
UPDATE.......SC
DISCUSSION...JVC/TF
MARINE.......DG


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